[bgp-config] add networking_bgp_config_update endpoint

[nicolaskagami]

Adds a networking_bgp_config_update endpoint with the following updatable fields:

pub struct BgpConfigUpdate {
    #[serde(flatten)]
    pub identity: IdentityMetadataUpdateParams,

    /// Update the BGP announce set associated with this configuration.
    pub bgp_announce_set_id: Option<NameOrId>,

    /// Update the maximum number of equal-cost paths.
    pub max_paths: Option<MaxPathConfig>,
}

• vrf isn't added because it's to be removed.
• asn isn't included because it may be used as an id internally.

closes #10066

[nicolaskagami]

(This is my first PR as an Oxide employee, so I'm still learning the ropes)

This was made drawing from Levon's old PR and the neighboring bgp-config methods. I changed a couple of things to improve readability (e.g. repeated code, some clone()s) and introduced the use of IdentityMetadataUpdateParams, which seems to be standard.

I also didn't attempt to remove the vrf field from the create / etc, as that might have other implications. Would we like to deprecate it first? Should we jump straight to failing vrf-containing calls?

Lastly, I wrote a little unit test mostly because it simply pains me not to do it. It was great because I found a (rather innocuous) bug on bgp_config_get (#10609) and learned some more about how things work.