Add headers for parse_csv

[vladimir-dd]

As a follow-up on vectordotdev/vector#5371 we should add an option to set headers for the parse_csv function, for example:

parse_csv!("foo,bar", headers: ["header1", "header2"])
// { "header1": "foo", "header2": "bar" }

[jszwedko]

Discord user asking for this: https://discord.com/channels/742820443487993987/746070591097798688/834837811214418021

[jerome-kleinen-kbc-be]

Similarly this could be relevant for parse_tokens. You could also consider generalising this using a VRL function that takes two arrays, one with keys and one with values, that returns an object. This generic function could do some checks on array lengths etc.

[bruceg]

Noting here that the PRs associated with #1157 solve most of this, though it may be more efficient to handle it in parse_csv directly.