Upgrade bitflags to v2.3

[rsdlt]

Include bitflags v2.3 in axum/Cargo.toml.
Derive required macros for EventFlags and MethodFilter structs.
Run cargo check / build / test to validate.

Refs: #2066

Motivation

Motivation explained in Refs: #2066

Solution

Upgrade bitflags to v2.3 in axum/Cargo.toml.
Derive only the required macros in bitflags v2+ for EventFlags and MethodFilter structs.
Run cargo check / build / test to validate.

[davidpdrsn]

Hm I'm not a fan of how it makes bitflags part of our public api with the PublicFlags type:

I have been wondering if we should just ditch bitflags and define our own type as

struct MethodFilter {
    delete: bool,
    get: bool,
    head: bool,
    options: bool,
    patch: bool,
    post: bool,
    put: bool,
    trace: bool,
}

Then we could also expose just the methods we think users need, instead the kitchen sink that bitflags generates.

[rsdlt]

Hm I'm not a fan of how it makes bitflags part of our public api with the PublicFlags type:

I have been wondering if we should just ditch bitflags and define our own type as

struct MethodFilter {
    delete: bool,
    get: bool,
    head: bool,
    options: bool,
    patch: bool,
    post: bool,
    put: bool,
    trace: bool,
}

Then we could also expose just the methods we think users need, instead the kitchen sink that bitflags generates.

I think that's a better approach. An added benefit is not having to worry about breaking changes from bitflags in the future if they happen.

I suggest we close this PR and I'll try to tackle it this way.

Cheers.