add onchain stamps to examples

[jmcook1186]

Adds source code for onchain Stamps demo app to /examples directory (accompanies tutorial at docs.passport.gitcoin.co)

[nutrina]

This is really great. Thanks for putting this together.
Some small improvement to help first-time impatient devs like me:

[nutrina]

I would also explain how one can check for his attestations in the EAS explorer. For example:

• You can check your EAS attestations here:
  https://base-goerli.easscan.org/address/ 0x85F01cfF157199527528788ec4A6336615C989
• The Passport Attestation schema is:
  Oxe496278adc2c09ec932359fdfb015ca7aae61260fbfa07f6d46eef9cf707b8 ( https://base-goerli.easscan.org/schema/view/ Oxe496278adc2c09c932359dfb015ca7aae61260fbfa07f6d46eef9cf707b8 ) and the trusted attester (from) is 0x5bbBC73312506834c40A90066F29FFb8200

[lebraat]

@erichfi -- This is ready to be merged