Replace bool with enum for a function parameter of label::fmt_string (#1875)

* Replace bool with enum in label::fmt_string

This commit addresses code smell where the said function takes a bool
to decide whether it percent-encodes the UTF-8 encoding of the given
string. For clarity, we define an enum EncodingStrategy so that we
can see the callers' intent at the call sites by reading the variant
names rather than true/false.

* Update CHANGELOG.next.toml

* Update CHANGELOG.next.toml

Co-authored-by: John DiSanti <jdisanti@amazon.com>

* Make test use updated function signature of fmt_string

This commit modifies a call site for the updated function signature of
`aws_smithy_http::label::fmt_string`, otherwise the test would fail to
compile, leading to a failure in CI.

Co-authored-by: Saito <awsaito@c889f3b5ddc4.ant.amazon.com>
Co-authored-by: John DiSanti <jdisanti@amazon.com>
Co-authored-by: Zelda Hessler <zhessler@amazon.com>