fix(ssz): Add validation checks for BitList and BitVector

[kevaundray]

For my understanding:

BitVector

• BitVector[N] is a fixed-length bit array that is serialized into ceil(N/8) bytes. Any bits above position N should be 0 since they are just padding. The old code seemed to just truncate and not check this.

• There also were missing checks on the byte count. For example BitVector[5] needs 1 byte., however the code seemed to allow passing for example 2 bytes.

BitList

• BitList[N] is variable length. We use a sentinel 1 bit after the data to encode the actual length. This means that the serialized form should always have at least one byte (for the sentinel bit) and moreover, the last byte should always be non-zero since it holds the sentinel.

Changes

• This adds the necessary validation checks for bitlist and bitvector

Related to #10793

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