proxy: rest based light client

[chirag-parmar]

meta: #3101

Description:

This PR ports the existing light client within nimbus-eth2 for use in the verified proxy with light client updates downloaded purely via the REST API. This is beneficial for using the proxy in a browser. In doing so, the PR modifies the behaviour of the light client slightly as noted below.

Notes:

1. Syncing: The original light client starts with a bootstrap and syncs blocks from bootstrap to head. It then follows the chain optimistically. here and here. The verified proxy version of the light client uses the bootstrap update and subsequent sync committee updates only for establishing the (current and next) sync committee. Once established, however, it starts following the chain directly from the current head. From the perspective of trust, optimistically following the chain from the current head is the same as syncing the chain and then following it optimistically (since the validation is only dependent on the current sync committee which doesn't change within the current sync committee period) here

2. Peers and Endpoints: The original light client uses multiple peers concurrently to download a particular update. The verified proxy version uses the same code but abstracts out rotating/randomizing peers (in the case of NVP it is API endpoints) to the backend implementation through the use of reqIds. The modified light client uses unique request ids for all concurrent requests which can then be used to rotate/randomize and score API endpoints. The parallelization parameter for the light client is hardcoded to 2.

3. Downloading Updates: The original light client used a mix of light client updates and p2p gossip to maintain the canonical chain. The verified proxy version purely uses light client updates to maintain the canonical chain. Hence, it runs the manager loop on a per slot frequency rather than the per sync committee period frequency of the original client(not fully true, it is quite arbitrary but can be considered the average case scenario)

Other Changes:

• remove deprecated warnings
• remove extra file (c_frontend)
• fix nits (FullVersionStr)