Dedicated dynamic path query syntax in VRL

[JeanMertz]

This issue got spun out from this comment.

Dynamic field access in VRL is currently supported through the get function.

We want to remove this function in favour of dedicated syntax to achieve the same result.

Specifically, we would like to allow this:

idx = 12
bar = "baz"
.foo."{{bar}}"[idx]

That is, we'll use string-templating (see vectordotdev/vector#8467) to allow dynamic fields, and allow variables (that must resolve to a number) inside [...] for array indexing.

[JeanMertz]

One thing I realized, is that there's one feature of the get function, which isn't covered by this dynamic path lookup syntax, which is support for dynamically sized arrays for nested path access.

That is, this works:

{ "my_path": ["foo", "bar", 0], "foo": { "bar": [{"baz": true }] } }

path = push!(.my_path, "baz")

get!(., path)

true

This can't be achieved using the syntax proposed in this issue. There are ways to make this work, but I'll have to think some more on what the most promising approach would be. It might also be the case that this is something we don't want to support right now, maybe?