no way to add two bivariates polynomials (esp w/ different max degrees)

https://github.com/oasisprotocol/oasis-core/blob/18da8bc49d92a543795246b8bb7fb574df070eec/secret-sharing/src/poly/bivariate.rs#L15

while we don't need to do proactive verifiable secret sharing by adding zero holed bivariate polynomials since we can do churp dimension switching and get the same effect, it will be useful to be able to add a zero holed bivariate polynomial to a bivariate used for churp, esp with the max degree(s) growing.

the rationale for wanting this ability is this: we would like to be able to optionally grow the reconstruction threshold (as well as the number of shareholders for availability) as the value of the secrets being protected grows. in the context of a blockchain system, this would correspond to something like the total value locked increasing over time as the usage increases.

Start with secret-sharing/src/poly/bivariate.rs at the linked implementation and inspect how bivariate polynomials represent their maximum degrees and coefficients. Define the addition behavior for zero-holed polynomials with different maximum degrees, including the Churp use case; done means those polynomials can be added safely as degrees grow and the behavior is covered by tests.