CDRIVER-5589 add option to prefer TCP for SRV lookup

[kevinAlbs]

Summary

Add option to prefer TCP for SRV lookup.

Checks for the environment variable MONGOC_SRV_PREFER_TCP=ON.

Verified with this patch build.

Background & Motivation

See DRIVERS-2919 for a description of the problem this is intended to solve. DRIVERS-2919 proposes a solution for all drivers. This PR is intended as a libmongoc-only solution for the problem impacting HELP-59749.

Rejected alternatives

Adding an option setter

Initially considered was adding an option setter:

MONGOC_EXPORT (void)
mongoc_client_pool_set_srv_prefer_tcp (mongoc_client_pool_t *pool, bool value);

SRV lookup first occurs in mongoc_client_pool_new_with_error. An option setter like mongoc_client_pool_set_ssl_opts requires a client pool to already be constructed.

Adding a new constructor

Also considered was a new mongoc_client_pool_new_with_opts constructor and new options struct:

MONGOC_EXPORT (mongoc_client_pool_opts_t *)
mongoc_client_pool_opts_new (void);

MONGOC_EXPORT (void)
mongoc_client_pool_opts_destroy (mongoc_client_pool_opts_t *opts);

MONGOC_EXPORT (void)
mongoc_client_pool_opts_set_srv_prefer_tcp (mongoc_client_pool_opts_t *opts, bool value);

MONGOC_EXPORT (mongoc_client_pool_t *)
mongoc_client_pool_new_with_opts (const mongoc_uri_t *uri,
                                  mongoc_client_pool_opts_t *opts,
                                  bson_error_t *error) BSON_GNUC_WARN_UNUSED_RESULT;

However, this breaks established patterns for passing options to the client pool. I prefer to avoid a significant API addition.

Adding a URI option

Also considered was adding a URI option srvPreferTCP. This may result in a non-portable URI connection string. URIs are intended to be portable among drivers:

Connection Strings should be portable

[vector-of-bool]

LGTM

[eramongodb]

char *MONGOC_EXPERIMENTAL_SRV_PREFER_TCP -> char *srv_prefer_tcp (not a macro).