Add `BuilderApi` multiplexer

[ltitanb]

We should support use cases where users need to send some requests in the builder API to different sidecars based on the validator pubkey, which allows users to run a single beacon client (which generally support a single BuilderAPI endpoint).

In practice this should be a mapping of Array<BlsPublicKey> -> PbsModuleConfig in the main config file, so that users can customize eg. relays and different images to run.

[aimxhaisse]

Bunch of ideas for more advanced filtering:

• by lido operator id (i.e: commit-boost would fetch keys from the Lido smart-contract) ; this is useful because Lido has as a whitelisted set of relayers to use,
• by SSV operator id (i.e: commit-boost would fetch keys from the SSV smart-contract),
• by withdrawal credentials (i.e: to have a specific customer target specific relays).

Tricky part of this is: how to keep the mapping updated, for instance, if a new key is added to a setup ; this implies to have a way to automatically reload the mapping in real-time (or periodically restart commit-boost and accept a lag).

[aimxhaisse]

Example of such a "hypothetical" config:

      strategies:
        - name: default
          relays: ['holesky-titan', 'holesky-bloxroute', 'holesky-aestus', 'holesky-eden', 'holesky-ultrasound']
        - name: lido-holesky
          relays: ['holesky-flashbots']
          filters:
            lido_operators: [8]
        - name: lido-ssv
          relays: ['holesky-titan', 'holesky-flashbots', 'holesky-eden']
          filters:
            ssv_operators: [199]
        - name: primev
          relays: ['holesky-titan']
          delaying: False
          filters:
            public_keys:
              - 88f51c0193d5b3f0ff408ea82e57ba9d1c19206b6afa7a55fde4580f326cc055fb407fdf37bdb22bf6ddf533ec218e7d
              - 88f51e63fb82120c9a405b4cb03e45d922a91884fcaec3ef2426eecf25c70695dca8806731a082b746fe4673a28b2155
              - 88f56fa47d863ecc1050eaa9eae86bbc1987490d8b7a66cf9bf69727aea603b3ca90bdbb2d79de8411cea280838f6800
              - 88f598d3ff3a1c9659f795d851c79dafd6c3bac12f55f87221da3127059b4223bb0398ee599802351ab95134baa7b318
              - 88f5a12d7570bcaab54a3b6e477cabfad247158c70a8eb286974ce6e01f2b68f785092ddcea185510f16dc8a29c64e5b
              - 88f5a38897fe6c0f423ea19c3d28460e104e8d73de4220581f7842ab34907f4bf7109c3a1e1c6cf9f85a915c70cea5be