Add pricing json file

[sophie200]

Why are these changes needed?

To have the arena's models pricing information open for public collaboration

What the changes are

Added a pricing json file that has each of the arena's models input token price (per million tokens), output token price (per million tokens), and the source where we got the information

The models that aren't listed in the json file are models where we couldn't find any pricing or model size info

Related issue number (if applicable)

Checks

• I've run format.sh to lint the changes in this PR.
• I've included any doc changes needed.
• I've made sure the relevant tests are passing (if applicable).