GetExchangeCodes is missing from the library

https://docs.alpaca.markets/reference/stockmetaexchanges-1

Need to add a new `marketdata.Client` method.

Two potential approaches:
- call it `GetExchanges` and return `[]Exchange` (requires a new `Exchange` entity with e.g. `Code` and `Name` fields)
- call it `GetExchangeCodes` and return `map[string]string` (with the code as the key and the name as the value)

I prefer `GetExchanges` but `GetExchangeCodes` might fit better with the library.

@gnvk what do you prefer?

Example API response body:

```json
{
"A": "NYSE American (AMEX)",
"B": "NASDAQ OMX BX",
"C": "National Stock Exchange",
"D": "FINRA ADF",
"E": "Market Independent",
"H": "MIAX",
"I": "International Securities Exchange",
"J": "Cboe EDGA",
"K": "Cboe EDGX",
"L": "Long Term Stock Exchange",
"M": "Chicago Stock Exchange",
"N": "New York Stock Exchange",
"P": "NYSE Arca",
"Q": "NASDAQ OMX",
"S": "NASDAQ Small Cap",
"T": "NASDAQ Int",
"U": "Members Exchange",
"V": "IEX",
"W": "CBOE",
"X": "NASDAQ OMX PSX",
"Y": "Cboe BYX",
"Z": "Cboe BZ"
}
```