feat: support for ellmer tools

[JohnCoene]

Adds support for ellmer::tool.

# create an ellmer tool
current_time <- ellmer::tool(
  \(tz = "UTC") {
    format(Sys.time(), tz = tz, usetz = TRUE)
  },
  "Gets the current time in the given time zone.",
  tz = ellmer::type_string(
    "The time zone to get the current time in. Defaults to `\"UTC\"`.",
    required = FALSE
  )
)

mcp <- new_server(
  name = "R Calculator Server",
  description = "A simple calculator server implemented in R",
  version = "1.0.0"
)

# register ellmer tool with mcpr server
mcp <- add_capability(mcp, current_time)

serve_io(mcp)

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