Add vtadmin-web build flag for configuring fetch credentials

[doeg]

Signed-off-by: Sara Bee 855595+doeg@users.noreply.github.com

Description

As the title says, this adds a build flag for vtadmin-web to configure the fetch credentials property when making requests against vtadmin-api. This is useful for VTAdmin deployments that use cookie-based authentication.

Related Issue(s)

N/A

Checklist

• Should this PR be backported?
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

N/A

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving
• VReplication
• Cluster Management
• Build/CI
• VTAdmin

[doeg]

Huh... closing the pull request was unexpected. 🤔 I can't seem to re-open this PR against the same (downstream) branch, because... git fork reasons? I'll open a new one. Sorry about that!

I believe what happened is:

• I did git reset --soft HEAD^ to update a comment
• I neglected to git commit and force-pushed an empty diff to this branch
• GitHub automatically closed the PR. 🤔 ...thanks?
• Subsequent pushes to tinyspeck:sarabee-vtadmin-fetch-credentials weren't picked up by this PR. (It's possible the PR needed to be re-opened, but I didn't want to spam my reviewers' emails with my "experimentation".)

...so, nothing see here. 🙈