QueryResult: use stdlib for envelope decoding

[liggitt]

cc @bwplotka

part of #2105

1. Adds a benchmark for QueryResult decoding
2. Switches to stdlib for the outer envelope decoding only (benchmarks show the json-iterator-optimized secondary decodings are still faster than plain stdlib, will address those in follow-ups)

Benchmark results of switching the envelope decode to stdlib:

For scalar query results (tiny)

• cpu is worse on go 1.26 and 1.27, but still nanoseconds,
• allocs and bytes are worse on go 1.26, better on go 1.27

For vector and matrix query results (large)

• cpu is worse on go 1.26, better or neutral on go 1.27
• allocs and bytes are better on go 1.26 and go 1.27

goos: darwin
goarch: arm64
pkg: github.com/prometheus/client_golang/api/prometheus/v1
cpu: Apple M4 Pro
                                │ main-jsoniter │              go1.26.6              │              go1.27.1              │
                                │    sec/op     │   sec/op     vs base               │   sec/op     vs base               │
QueryResult/scalar-14               473.4n ± 4%   731.7n ± 2%  +54.58% (p=0.002 n=6)   611.6n ± 1%  +29.22% (p=0.002 n=6)
QueryResult/vector-14               74.66µ ± 9%   86.01µ ± 1%  +15.20% (p=0.002 n=6)   63.21µ ± 1%  -15.33% (p=0.002 n=6)
QueryResult/matrix-float-14         19.59µ ± 7%   26.84µ ± 2%  +37.02% (p=0.002 n=6)   19.69µ ± 1%        ~ (p=0.589 n=6)
QueryResult/matrix-histogram-14     168.8µ ± 3%   268.5µ ± 1%  +59.11% (p=0.002 n=6)   166.3µ ± 2%        ~ (p=0.132 n=6)
geomean                             18.49µ        25.95µ       +40.37%                 18.86µ        +2.02%

                                │ main-jsoniter │              go1.26.6               │              go1.27.1               │
                                │     B/op      │     B/op      vs base               │     B/op      vs base               │
QueryResult/scalar-14                544.0 ± 0%     704.0 ± 0%  +29.41% (p=0.002 n=6)     184.0 ± 0%  -66.18% (p=0.002 n=6)
QueryResult/vector-14              38.04Ki ± 0%   37.88Ki ± 0%   -0.43% (p=0.002 n=6)   24.54Ki ± 0%  -35.48% (p=0.002 n=6)
QueryResult/matrix-float-14        21.33Ki ± 0%   21.24Ki ± 0%   -0.41% (p=0.002 n=6)   17.86Ki ± 0%  -16.28% (p=0.002 n=6)
QueryResult/matrix-histogram-14    133.2Ki ± 0%   131.5Ki ± 0%   -1.23% (p=0.002 n=6)   128.5Ki ± 0%   -3.51% (p=0.002 n=6)
geomean                            15.48Ki        16.42Ki        +6.11%                 10.03Ki       -35.20%

                                │ main-jsoniter │             go1.26.6              │              go1.27.1              │
                                │   allocs/op   │  allocs/op   vs base              │  allocs/op   vs base               │
QueryResult/scalar-14                16.00 ± 0%    17.00 ± 0%  +6.25% (p=0.002 n=6)    10.00 ± 0%  -37.50% (p=0.002 n=6)
QueryResult/vector-14                747.0 ± 0%    689.0 ± 0%  -7.76% (p=0.002 n=6)    670.0 ± 0%  -10.31% (p=0.002 n=6)
QueryResult/matrix-float-14          497.0 ± 0%    459.0 ± 0%  -7.65% (p=0.002 n=6)    410.0 ± 0%  -17.51% (p=0.002 n=6)
QueryResult/matrix-histogram-14     4.567k ± 0%   4.229k ± 0%  -7.40% (p=0.002 n=6)   4.180k ± 0%   -8.47% (p=0.002 n=6)
geomean                              405.8         388.3       -4.32%                  327.3       -19.34%

[bwplotka]

Thanks!

[liggitt]

rebased