fix(Examples): Validate EC on decrypt

[lucasmcdonald3]

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Description of changes:

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[RitvikKapila]

This looks good overall.
Two things:
First, do we not want to make the changes of providing the encryption context during decrypt in other migration examples? For instance:

• migration_raw_rsa_key_example.py
• migration_raw_aes_key_example.py
• migration_aws_kms_key_example.py

I think we should, to point out explicitly that they should provide the encryption context while calling decrypt.

Second, I left another comment about the wording of the comment when providing encryption context.

[RitvikKapila]

According to Ryan, we want to tell the customer to always provide the encryption context during decrypt. I wonder if it'll be better if we remove the word "Verify" and just say "Enter the encryption context supplied to the encrypt method".

Same goes for Line #192 in this file and all other examples.

[lucasmcdonald3]

Updated, I slightly preferred "Provide" instead of "Enter"

Good catch on migration examples, I updated those to supply EC on decrypt if decrypting with keyring.
I also added manual EC validation if decrypting with MKP since MKPs don't support EC validation on decrypt.

[RitvikKapila]

very small nit: I would change the name of the encrypted header, e.g., enc_header_mkp to encrypted_header_mkp just because we have the decrypted header variable as decrypted_header_keyring_using_mkp. However, it's not a big thing if we keep this as is.

Otherwise this LGTM. Approved.