Address OOM risk in witness parsing

[IITI-tushar]

Refers #41
This pull request refactors the way witness files and data are parsed in the pkg/attestation/parser.go file. The main improvement is the introduction of a unified parsing function that works with any io.Reader, which simplifies handling both files and in-memory data. The code now also checks for trailing JSON and improves error handling for malformed input.

Refactoring and improved parsing logic:

• Introduced a new parseWitnessReader function that parses attestation data from any io.Reader, allowing both file-based and in-memory parsing to share the same logic. (pkg/attestation/parser.go)
• Updated ParseWitnessFile and ParseWitnessData to use the new parseWitnessReader function, reducing code duplication and improving maintainability. (pkg/attestation/parser.go)

Robustness and error handling:

• Added a check for trailing JSON after the main attestation object, returning an error if extra data is found, which helps catch malformed input files. (pkg/attestation/parser.go)
• Improved error messages and handling for missing or malformed payloads and predicates in the attestation JSON. (pkg/attestation/parser.go)