change: AssetPair API refactoring

[testinginprod]

I propose to refactor AssetPair.

Remove it from proto files.

Make it a custom proto type we can use in place of string.

Make the AssetPair API safe to use, by unexporting fields.

Current problems:

• pair somewhere is defined as string, somewhere else as AssetPair.
• code is cluttered by conversion to AssetPair to string, and string to AssetPair
• in some part of the code we use token0, token1
• Validation is needed everywhere because engineers are unsure when the AssetPair is safe to use.

Example implementation:

type AssetPair struct {
    t0 string // Fields are private to avoid bad initialization
    t1 string
}

func (p AssetPair) Token1() string { return p.t1 }

func (p AssetPair) String() string { ... } 

func NewAssetPair(token1, token2 string) (AssetPair, error)

[AgentSmithMatrix]

@NibiruHeisenberg @Unique-Divine thoughts.

[Unique-Divine]

This was completed successfully. Currently, the asset.Pair type is a type alias for string with functions that make it feel like we're working with a struct.