Prevent the use of uppercase letters in HeaderName::from_static

[rylev]

Hopefully preventing issues like that in #1048 once and for all.

This is the error you now get if you try to use an uppercase letter in a headername:

error[E0080]: evaluation of constant value failed
   --> sdk/core/src/headers/mod.rs:204:13
    |
176 |         ensure_no_uppercase(s);
    |         ---------------------- inside `headers::HeaderName::from_static` at sdk/core/src/headers/mod.rs:176:9
...
204 |             panic!("header names must not contain any uppercase letters");
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |             |
    |             the evaluated program panicked at 'header names must not contain any uppercase letters', sdk/core/src/headers/mod.rs:204:13
    |             inside `ensure_no_uppercase` at /home/rylevick/.rustup/toolchains/1.63.0-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/panic.rs:57:9
...
265 | pub const ACCEPT: HeaderName = HeaderName::from_static("Accept");
    |                                --------------------------------- inside `headers::ACCEPT` at sdk/core/src/headers/mod.rs:265:32
    |
    = note: this error originates in the macro `$crate::panic::panic_2021` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0080`.
error: could not compile `azure_core` due to previous error

[bmc-msft]

Can this be used in HeaderName::from_cow as well?

[rylev]

We could but the error message is ever so slightly worse so I'm not so sure if it's the right move.