Fix aws-sigv4 canonical request formatting fallibility (#1656)

* Fix panic occurred in formatting CanonicalRequest

This commit addresses panic in the case of formatting a CanonicalRequest
containing invalid UTF-8 in the header value.

The commit aims for the least disturbance to the codebase in order to
pass a given failing test in the PR. We want to quickly determine if
this low-cost approach addresses the problem before we commit ourselves
to start refactoring CanonicalRequest to have a special format function.

Fixes #711

* Update test_signing_utf8_headers to proptest

This commit converts test_signing_utf8_headers to a proptest. The
original test only specified hardcoded non-UTF8 bytes in a request
header. To ensure that `crate::http_request::sign` does not panic no
matter what valid byte sequence for HeaderValue is, the proptest covers
a wider range of inputs.

For consistency, the test has been moved from `canonical_request.rs` to
`sign.rs`.

* Add InvalidHeaderError to make the error explicit

This commit introduces an error type InvalidHeaderError to indicate that
we ran into a problem in handling a HeaderValue within CanonicalRequest.

The error type contains a source error such as Utf8Error so a diagnostic
message can be printed if needed.

* Remove InvalidHeaderError for error refactoring

This commit effectively reverts 739b32c. Knowing that we will be cleaning
up error types, having InvalidHeaderError is too narrow a solution and
does not add value to the codebase.

* Update CHANGELOG.next.toml

Co-authored-by: Russell Cohen <rcoh@amazon.com>
Co-authored-by: Saito <awsaito@c889f3b5ddc4.ant.amazon.com>