add wallet balance endpoint

[tsachiherman]

What ?

This PR adds the balance endpoint for the xps-gateway.

Implementation

Quite straight-forward; just query the provider for the wallet balance.

Closes

Closes #23.

Open question

While this implementation would work just fine, it might be adventitious to change the

pub struct WalletBalance {
    /// The balance for the wallet; formatted as "5.32 ETH"
    #[serde(rename = "balance")]
    pub balance: String,
    /// The unit used for the balance; formatted as "ETH"
    #[serde(rename = "unit")]
    pub unit: String,
}

into something like this:

pub struct WalletBalance {
    /// The balance for the wallet; just the number itself.
    #[serde(rename = "balance")]
    pub balance: U256,
    /// The unit used for the balance; formatted as "ETH"
    #[serde(rename = "unit")]
    pub unit: String,
    /// The balance, formatted for display ; formatted as "5.32 ETH"
    #[serde(rename = "display_balance")]
    pub display_balance: String,
}

This would allow us to make the RPC more friendly for code-oriented users while retaining the user-facing formatter.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (470c257) 89.85% compared to head (0a79650) 91.16%.

Files	Patch %	Lines
xps-gateway/src/rpc/methods.rs	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
+ Coverage   89.85%   91.16%   +1.31%     
==========================================
  Files          12       12              
  Lines         355      419      +64     
==========================================
+ Hits          319      382      +63     
- Misses         36       37       +1     

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[insipx]

I think the more idiomatic way would be doing something like this:

pub struct WalletBalance {
    /// The balance for the wallet; just the number itself.
    #[serde(rename = "balance")]
    pub balance: U256,
    /// The unit used for the balance; formatted as "ETH"
    #[serde(rename = "unit")]
    pub unit: String,
}

with a custom display implementation:

impl Display for WalletBalance {
// ...
}

I would also be in favor of turning unit into an enum,

pub struct Unit {
    Eth,
    Other(String),
}

Since we are mostly going to be using Eth, and down the line we can add other chains we use

[insipx]

just a small nit, if we unwrap in a method used by the RPC our server will go down

Otherwise looking good! Ready to approve after that is turned into an error

[jac18281828]

This looks like a good start. Make sure to address those codecov warnings about lack of tests.