EIP-7594 - Update ENR record

[nalepae]

Proposal for #3744

The maximum encoded size of a node record is 300 bytes. (Source).

1. Field key

Reducing the custoding_subnet_count key to csc saves 19 bytes.

2. Field value type

Switching the value of custoding_subnet_count from SSZ uint64 to big endian integer has multiple advantages:

• ENR are already RLP encoded, so there is no need to both SSZ and RLP encode.
• RLP encoding stores positive integer efficiently. If a positive integer is less than 128 (which is likely to be the case for the subnets count), then the value needs exactly one byte of storage when encoded. (Source).

Switching the value of custoding_subnet_count from SSZ uint64 to big endian integer saves 7 bytes.

Total

Changing 1. and 2. saves a total of 19 + 7 = 26 bytes. 26 bytes represents about 8.5% of the maximum size of an encoded node record.

[ppopth]

Looks good to me

[hwwhww]

lgtm