Store delegation token hmac as base64

[rukai]

This PR completes a TODO left in the code from #1626

Delegation tokens are created by a super user and allows a connection to kafka to authenticate as a specific user.
Shotover creates them to allow proxying scram connections which would otherwise be unable to be proxied.

The token task in scram_over_mtls.rs creates delegation tokens when the transforms request them.
It will cache the token and fetch new tokens if they are not in the cache yet.

The tokens are stored in a type called DelegationToken.
This contains the token_id which acts as the tokens username and the hmac which acts as the tokens password.

shotover-proxy/shotover/src/transforms/kafka/sink_cluster/scram_over_mtls.rs

Lines 328 to 333 in c1331f8

	#[derive(Clone)]
	pub struct DelegationToken {
	pub token_id: String,
	// TODO: store as base64 string
	pub hmac: Vec<u8>,
	}

Currently the hmac field stores the raw bytes of the hmac, in a Vec<u8>.
This PR changes the hmac field to instead store the base64 encoding of the hmac, in a StrBytes.

This is more efficient as we dont need to convert to base64 every time we use the hmac to login with token.

[codspeed-hq]

CodSpeed Performance Report

Merging #1628 will not alter performance

_{Comparing rukai:store_hmac_as_base64 (d7ec227) with main (50b6301)}

Summary

✅ 37 untouched benchmarks