feat(stdlib): Add timezone option for parse_timestamp function

[titaneric]

Summary

#962 mentions the possibility of adding timezone optional argument for parse_timestamp function. I find it it's rather easy to implement it, so I would give it a try.

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

How did you test this PR?

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on
  our guidelines.
• No. A maintainer will apply the "no-changelog" label to this PR.

Checklist

• Our CONTRIBUTING.md is a good starting place.
• If this PR introduces changes to LICENSE-3rdparty.csv, please
  run dd-rust-license-tool write and commit the changes. More details here.
• For new VRL functions, please also create a sibling PR in Vector to document the new function.

References

Closes: #962

[pront]

This looks good, thank you @titaneric. You are on a roll 🔥

Let's also prepare a sibling Vector PR to explain this new parameter.

[titaneric]

I have turned on the related PR and it's ready to review at vectordotdev/vector#22121.