Add --http-token-path flag to the VC

[michaelsproul]

Description

We considered the issue of VC HTTP token provisioning solved with the closure of:

• Allow provisioning of the api-token.txt file (HTTP server auth) #3446
• Remove VC response signing #5423

However, we didn't add a CLI flag to set the token path. This issue tracks the addition of this flag.

Present Behaviour

Lighthouse always reads and writes the token from the $datadir/validators/api-token.txt path.

Expected Behaviour

If the user supplies the --http-token-path to the VC then the token should be read/written from this path instead. The flag should require the --http flag to also have been provided.

[syjn99]

Hi, i'd like to take this one for my first contribution

[michaelsproul]

Awesome, thanks!

After you've got the code change worked out, you can add a CLI test in lighthouse/tests/validator_client.rs