An example contract for Solana

[yhzhang0128]

I have been working with Ali on this example contract. Comments are welcome and I will address them before merge.

[jayantk]

One big thing I think we should definitely do is check the pyth account keys in the instruction. People are going to essentially copy-paste this code, so we should make sure that it's complete and secure.

Aside from that, I Ieft some minor comments around documentation etc.

[jayantk]

Oh also you should install the pre-commit hooks here https://github.com/pyth-network/pyth-sdk-rs#pre-commit-hooks to fix this CI issue ^

[yhzhang0128]

Following Jayant's comments, I made the following changes:

1. Only the admin can initialize the loan information
2. Loan2Value will check the Pyth accounts assuming the admin gave correct information
3. Add more text in README about where/how Pyth SDK is used
4. Use confidence interval in the example
5. Test permission control by adding an unauthorized invocation of Init in the client

[jayantk]

awesome! I left a bunch of minor comments. I think you're getting pretty close though, so I suggest merging this PR and then addressing the comments in separate small PRs.

[yhzhang0128]

Hi Ali and Jayant, I resolved all your new comments. Specifically, I

1. make quantities as client-side parameters instead of hard-coded value;
2. use borsh serialization/deserialization for state.rs;
3. update the value calculation using integers instead of floating points;
4. resolve some renaming and small issues.

I also followed Ali's suggestion and rebase my branch with the latest main. I guess two things worth another eye:
(1) the value calculation in processor.rs;
(2) This PR now says 21 files changed and I'm a bit confused, because it seems to be comparing with the old main branch before I rebase with the latest main.