MONGOCRYPT-763 add in-place retry API

[mdb-ad]

Add in-place retry API to better support drivers that fan out KMS requests.

[kevinAlbs]

mongocrypt_kms_ctx_fail currently still refers to "network error" in error messages and increments the retry attempts:

ASSERT_OK(mongocrypt_kms_ctx_feed(kms, retryable_http), kms); // Increments kms->attempts.
ASSERT(mongocrypt_kms_ctx_should_retry(kms));
ASSERT_OK(mongocrypt_kms_ctx_fail(kms), kms); // Increments kms->attempts again.

I like the direction towards simplifying. Suggest resetting the KMS parser in set_retry. I expect this will reset the parser for both HTTP and network error and could further simplify the retry pattern:

while (true) {
    try {
        // TODO: Write and read from socket.
    } catch (e : NetworkError) {
        // Indicate network error:
        mongocrypt_kms_ctx_fail (kms);
    } finally {
        if (mongocrypt_kms_ctx_should_retry (kms)) {
            // Can retry due to a HTTP or network error.
            continue;
        }
    }
}

That way, mongocrypt_kms_ctx_fail does not change its meaning (still means only network error).

Testing this change in 456d523 appears to work.