Authorize endpoint: allow serial number without colons

#### Feature Description

In Consul's Connect Authorize endpoint, the documentation of `/agent/connect/authorize` path requires `ClientCertSerial` to be provided with "the colon-hex-encoded serial number".
See: https://www.consul.io/api/agent/connect.html#clientcertserial

While possibly making the serial number more human readable, the colon-hex encoding might have some drawbacks.

In particular, despite being common representation for Go-based clients, colon-hex encoding does not seem to be the default for popular OpenSSL conversion functions, such as `BN_bn2hex()`.
See: https://www.openssl.org/docs/man1.1.0/crypto/BN_bn2hex.html

It is true that colons can later be added as a delimiter to the output of OpenSSL functions such as `BN_bn2hex()`. However, it might be easier, more efficient and more elegant to rather make the server side accept serial numbers represented in hexadecimal encoding without colons.

IIUC, the server-side code that handles certificate serial numbers is not implemented yet.
See: https://github.com/hashicorp/consul/blob/b5abf61963c7b0bdb674602bfb64051f8e23ddb1/agent/agent_endpoint.go#L1232

Therefore, I would like to propose this enhancement to the API before the server-side code is implemented. In the meantime, this merely requires a modifying the docs.

#### Use Case(s)

This enhancement can be useful for Authorize endpoint REST clients which use OpenSSL, such as C/C++ clients.

Start with the referenced agent/agent_endpoint.go location and the /agent/connect/authorize documentation. Confirm whether ClientCertSerial handling exists and determine how colon-free hexadecimal serials should be accepted; done means the endpoint behavior and its documentation consistently describe and support the proposed representation.

Written by the indexing model from the issue text.