Add - Settings

[brentru]

This pull request allows users to select options such as gain, oversampling, measurement rate, power mode, etc. It replaces the fixed enum for AnalogIn Gain with the generic settings map, and uses the same settings map for I2C device configuration.

Specification
Design overview:

1. The definition is the source of truth for what settings are exposed for the pin/component
2. The sensor driver is only place where the index 2 of gain maps.
3. Protobuf never changes when a component is added, or when a new setting is added to an existing component.

Expected Operation:

1. Let's assume a user is in the New Component flow in IO.Adafruit.com
2. User selects BME280
3. User clicks "Advanced Options" and fills: Temperature Oversampling = 4x, PRessure Oversampling = 8x, Humidity oversampling = 2x.

Protobuf msg is filled using a settings field, which holds all of the device's setting configurations.

map<string, uint32> settings = 3;  

where:

    settings: {
      "temp_oversampling": 3,      // "4x"
      "pressure_oversampling": 4,  // "8x"
      "humidity_oversampling": 2   // "2x"
    }

4. Message is sent to the device
5. Firmware passes the map to the driver
6. Driver stores the raw index map
7. Driver interprets the raw index map during its begin() call

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Related: adafruit/Wippersnapper_Components#327
Resolves #126

[tyeth]

This looks great, thanks guys for being so amenable to the idea

[brentru]

Awesome, it was needed even if itll be a lift. Once I have the second approval I'll merge