Creating object oriented contexts

[kathrynfejer]

Related to #78

In order to help the eventual separation of xtt using tpm and xtt without a tpm, we could create a group_context that abstracts away the implementation of daa (software or tpm) that we use.

Proposed changes to xtt_client_group_context:

struct xtt_client_group_context {
    xtt_group_id gid;
    unsigned char basename[MAX_BASENAME_LENGTH];
    struct xtt_signing_context_{tpm OR software_daa}; //this is the "private key"
}

struct xtt_signing_context_tpm {
    union {
        xtt_daa_credential_lrsw lrsw;
    } cred;
    TPM_HANDLE key_handle;
    char key_password[MAX_TPM_PASSWORD_LENGTH];
    uint16_t key_password_length;
    TSS2_TCTI_CONTEXT *tcti_context;
}

struct xtt_signing_context_software_daa { //better name to come
    union {
        xtt_daa_credential_lrsw lrsw;
    } cred;
    union {
        xtt_daa_priv_key_lrsw lrsw;
    } priv_key;
}

This should decrease the number of #ifdefs we need.