fix(frontend-canister): allow overwriting default HTTP headers

[smallstepman]

Description

Let's use HashMap instead of Vec to aggregate headers, this will allow overwriting headers which are provided by default by asset canister, like Content-Type, Content-Encoding, Cache-Control.

Example .ic-assets.json config

[
    {
        "match": "web-gz.data.gz",
        "headers": {
            "Content-Type": "application/octet-stream",
            "content-encoding": "gzip" 
        }
    }
]

Notice

It's still impossible to set a custom ETag header since its value is set after user-configured headers (ignore let streaming_strategy =...):

sdk/src/canisters/frontend/ic-certified-assets/src/state_machine.rs

Lines 722 to 738 in 3680470

	if let Some(arg_headers) = asset.headers.as_ref() {
	for (k, v) in arg_headers {
	headers.insert(k.to_owned(), v.to_owned());
	}
	}
	let streaming_strategy = create_token(asset, enc_name, enc, key, chunk_index)
	.map(|token| StreamingStrategy::Callback { callback, token });
	let (status_code, body) = if etags.contains(&enc.sha256) {
	(304, RcBytes::default())
	} else {
	headers.insert(
	"ETag".to_string(),
	format!("\"{}\"", hex::encode(enc.sha256)),
	);
	(200, enc.content_chunks[chunk_index].clone())

This is desirable.

Example use-case

Some dapps require setting custom content-encoding and content-type.
Before this change, attempting to overwrite content-type via .ic-assets.json5 would cause duplicating HTTP header, e.g.

Metadata

closes https://github.com/dfinity/dx-triage/issues/152
continuation of dfinity/cdk-rs#282
requested by @zhangwei983

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration.

Checklist:

• The title of this PR complies with Conventional Commits.
• I have edited the CHANGELOG accordingly.
• I have made corresponding changes to the documentation.

[ericswanson-dfinity]

Can we have an e2e test for this?