RUST-2354 Add option for max length when parsing cstrings

[isabelatkinson]

The maximum length can be configured on an iterator or through new getter methods:

// document
let key = cstr!("aaaaa");
let doc = rawdoc! { key: "b" };
let iter = doc.iter().max_cstr_parse_len(key.len());
let b = doc.get_with_max_cstr_parse_len(key, key.len());

// array
let array = doc.get_array("array").unwrap();
let iter = array.into_iter().max_cstr_parse_len(5);
let val = array.get_with_max_cstr_parse_len(2, 5);

The API additions are exposed behind an sfp-internal feature flag. The documentation for them is hidden. This will give us more semver flexibility to make breaking changes if needed. Docs for the new features can be generated locally by running:

RUSTDOCFLAGS='-Z unstable-options --document-hidden-items' cargo +nightly doc --all-features --open

[kevinAlbs]

Looks good with a suggestion to validate regex.

[abr-egn]

(not needed for this PR since this is an internal API we can change if needed)

If we end up wanting to add any more control over parsing we might want to change this to get_with_options and make a proper options struct.

[patrickfreed]

Going to give this a quick try in monguard tomorrow to confirm we're not missing any requirements, but the interface LGTM after a quick look.