Used precise data type for price values

[pohlm01]

The documentation specifies using floats for price values. We tried to get the specification changed to use integers and a precision parameter instead to avoid confusion and possible rounding errors, but we did not succeed.
Therefore, we have to ensure that we have precise decimal values for all price-related arguments within the codebase (maybe rust_decimal) as well as in the DB (likely decimal data type).

Depends on #93

https://github.com/oadr3-org/openadr3-openapi/discussions/183 (non-public)