Write FRC describing Spark's requirements on miner SW (Curio, Venus)

[bajtos]

Document what Spark v2 requires from miner SW, so that projects like Curio and Venus have the specification of what they need to implement to support Spark.

See also:

• https://github.com/filecoin-project/FIPs/tree/master/FRCs

[bajtos]

Started a discussion: filecoin-project/FIPs#1086

[bajtos]

I wrote the first full version of the FRC, it's in the review process now: filecoin-project/FIPs#1089

I am closing this task as done.