Move GrafanaCfg to dedicated package

[wbrowne]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Compatibility

➜  gorelease -base=v0.274.0 -version=v0.275.0
# github.com/grafana/grafana-plugin-sdk-go/backend
## incompatible changes
ConvertFromProtobuf.GrafanaConfig: changed from func(map[string]string) *GrafanaCfg to func(map[string]string) *github.com/grafana/grafana-plugin-sdk-go/config.GrafanaCfg
ConvertToProtobuf.GrafanaConfig: changed from func(*GrafanaCfg) map[string]string to func(*github.com/grafana/grafana-plugin-sdk-go/config.GrafanaCfg) map[string]string
FeatureToggles: changed from FeatureToggles to github.com/grafana/grafana-plugin-sdk-go/config.FeatureToggles
GrafanaCfg: changed from GrafanaCfg to github.com/grafana/grafana-plugin-sdk-go/config.GrafanaCfg
GrafanaConfigFromContext: changed from func(context.Context) *GrafanaCfg to func(context.Context) *github.com/grafana/grafana-plugin-sdk-go/config.GrafanaCfg
NewGrafanaCfg: changed from func(map[string]string) *GrafanaCfg to func(map[string]string) *github.com/grafana/grafana-plugin-sdk-go/config.GrafanaCfg
PluginContext.GrafanaConfig: changed from *GrafanaCfg to *github.com/grafana/grafana-plugin-sdk-go/config.GrafanaCfg
Proxy: removed
SQLConfig: removed
WithGrafanaConfig: changed from func(context.Context, *GrafanaCfg) context.Context to func(context.Context, *github.com/grafana/grafana-plugin-sdk-go/config.GrafanaCfg) context.Context

# github.com/grafana/grafana-plugin-sdk-go/config
## compatible changes
package added

# github.com/grafana/grafana-plugin-sdk-go/live
## compatible changes
ScopeWatch: added

# summary
v0.275.0 is a valid semantic version for this release.